1. Field of the Invention
The present invention relates to a flat display apparatus, specifically to a shift register of a shift register array.
2. Descriptions of the Related Art
Currently, most Liquid Crystal Displays (LCD) are equipped with a gate driver and a source driver to generate a gate pulse signal and a data signal. However, due to the high cost of these drivers, alternative devices are proposed. For example, a shift register fabricated on a glass substrate with circuit functions equivalent to a gate driver, called integrated driving circuit, is one of the alternatives.
When applying the integrated driving circuit to a large sized liquid crystal display apparatus, a large amount of power is required. Consequently, a large sized Thin Film Transistor (TFT) is required at the output terminal of the shift register. Unfortunately, by using this approach, the output waveform of the shift register generates a coupling effect which causes the LCD to function abnormally.
FIG. 1 shows the architecture of a shift register 1 of the prior art. In this figure, CK represents a positive clock signal, XCK represents a negative clock signal, N−1 represents an input of the shift register 1, and N represents an output of the shift register. When the shift register 1 is applied to a large sized LCD, this design faces two serious defects. When the XCK signal rises, i.e. when the CK signal falls, the electric charge of the node P is released by the shift register 1, which causes a longer falling time of the output waveform N of the shift register 1. Referring to FIG. 2 of a timing diagram of the shift register 1, the dotted circle 21 and the dotted circle 22 indicate the rising of the XCK and the falling of the CK, respectively. At this moment, the electric charge of the node P drops immediately as shown in the dotted circle 23. FIG. 3 shows the output waveform of the shift register 1. According to the position indicated by the dotted circle 31, the falling time of the output waveform of the shift register 1 is about 12.4 μs. The falling time is too long for a LCD. Furthermore, when the size of the transistor 101 is enlarged, the coupling effect of the output terminal N becomes more significant, further causing erroneous operation of the next shift register, which considerably affects display quality.
According to the aforementioned problems and descriptions, the coupling effect and the long falling time of the output waveform are two common defects of current shift registers in LCDs. Consequently, designing a shift register to solve both problems remains an urgent topic of research.